NASA Hopeful for Sunday Shuttle Launch

CAPE CANAVERAL, Fla. - NASA engineers are trying to make up for lost time as they complete repairs on the space shuttle Discovery to prepare it to launch tomorrow.

Technicians fell up to four hours behind schedule today while replacing a suspect connector between the shuttle's external fuel tank and a hydrogen vent line, believed to be the source of a potentially dangerous leak that prevented the spacecraft?s Wednesday launch attempt.

Mission managers said today they expect the crews will be able to finish the fix in time for Discovery's planned liftoff attempt tomorrow at 7:43 p.m. EDT (2343 GMT) from the seaside Launch Pad 39A here at NASA?s Kennedy Space Center.

"We're good to go tomorrow, and we're looking forward to getting the count back running," said Mike Moses, who leads Discovery's mission management team, during a briefing here.

The weather outlook for tomorrow is encouraging, with an 80 percent chance of favorable conditions for launch, said shuttle weather officer Kathy Winters. A possible low cloud ceiling represents the main threat.

"That's a good weather situation, I think, for launch," Winters said.

Discovery is set for a 13-day mission to the International Space Station, including three spacewalks to install the last segment of the station's backbone-like truss and a new set of solar panels. The STS-119 mission will also drop off Japanese astronaut Koichi Wakata for a six-month stay at the space station, where he is due to replace NASA astronaut Sandra Magnus. Wakata will be Japan's first long-duration astronaut.

Initially slated to launch Feb. 12, Discovery?s mission was delayed nearly a month due to suspect fuel valves, which have since been replaced. After Wednesday?s launch delay, the mission was shortened from an original planned 14-day flight, featuring four spacewalks, to make way for an incoming Russian Soyuz spacecraft set to launch March 26.

If Discovery misses its chance to lift off on Sunday, then launching on Monday or Tuesday would require shortening the mission even more. If the shuttle cannot launch by Tuesday, NASA will stand down until April 7, after the Soyuz flight, to avoid overlap between the two vehicles at the space station.

No smoking gun

Engineers have not yet determined the root cause of the leak of flammable hydrogen gas, which poses an explosion risk so close to the shuttle. They are wrapping up work to replace a segment of the vent line to stop off the leak. The vent line helps keep the liquid hydrogen reservoir in Discovery?s external fuel tank adequately pressurized by siphoning excess gaseous hydrogen away as the super-chilled propellant boils off.

So far, tests suggest the new seal is holding, but NASA won't know for sure until the seal is subjected to the super-cold temperatures achieved when loading the fuel tank with cryogenic liquid hydrogen propellant before launch. If the leak persists, the launch will be scrubbed again until engineers can come up with a better solution.

"We like to know root cause for problems. I'm a little surprised? that we didn't find something more obvious," said shuttle launch director Mike Leinbach. "But from a launch safety, a crew safety perspective, we are fine."

If Discovery cannot launch tomorrow, NASA can try again on Monday and Tuesday, though the weather looks increasingly worrisome with each passing day. The current forecast predicts a 70 percent chance of good conditions on Monday, but drops to a 40 percent chance on Tuesday.

In the meantime, Discovery's seven-astronaut crew has been performing some final training exercises and resting up for their launch.

"The crew's in good shape," Moses said. "They've been keeping themselves busy doing some training. Hopefully they are going to take it easy and be ready to load into the ship tomorrow."

SPACE.com is providing continuous coverage of STS-119 with reporter Clara Moskowitz in Cape Canaveral and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed. Live Coverage begins at 2:30 p.m. EDT (1830 GMT) Sunday.

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